Add a fenced code block example in sample posts

The version of PHP markdown used supports fenced code blocks, but
because the marker is atypical (~ instead of `) it's a little more
subtle than Github's rendition.

This change documents that detail in the "highlights" blog post.
Add missing link target in sample post
Merge pull request #7 from ciarand/feature/add-fenced-code-blocks-example

Add a fenced code block example in sample posts